Carbon dioxide (CO2) is normally produced in the tissues of the human body where it becomes dissolved in the blood. The CO2 is then transported in blood to the lung where it diffuses across alveolar membranes and is expelled from the lungs during exhalation.
The term “capnography” refers generally to the measurement of CO2 in airway gas during the ventilation cycle. In patients who are undergoing anesthesia or mechanical ventilation, capnography is sometimes used to measure the partial pressure of CO2 (PCO2) at the airway opening during the ventilation cycle. During the inspiratory phase of the ventilation cycle (i.e., inhalation), a flow of inspired respiratory gas passes through the airway opening. Such inspired respiratory gas typically contains little or no CO2. Thus, during the inspiratory phase, the capnograph obtains an inspiratory baseline PCO2 measurement of zero. During the second phase of the ventilation cycle (expiratory upstroke), alveolar gas from the respiratory bronchioles and alveoli begins to pass out of the patient's airway and the capnogram measures a rapid increase in CO2 as the expiratory phase of the ventilation cycle proceeds. The third phase of the ventilation cycle is known as the “alveolar plateau,” during which a relatively constant PCO2 is measured at the airway opening. The PCO2 of the expired respiratory gas at the end of this third phase of the ventilation cycle (PETCO2) is typically of particular interest as it represents the last alveolar gas sampled at the airway opening during expiration. Finally, the fourth phase of the ventilation cycle is the inspiratory downstroke, during which the next inspiratory phase begins.
While these direct capnographic measurements at the airway opening do provide the clinician with important diagnostic information, the usefulness of such information is limited due to the fact that direct capnographic measurements of this type merely measure the partial pressure of CO2 without relating such measurement to the volume of respiratory gas that is passing through the airway opening as the measurement is taken. In view of this shortcoming of traditional capnography, it is now believed that a measurement of volume-normalized average alveolar PCO2 and pulmonary carbon dioxide elimination ({dot over (V)}CO2) are more clinically useful than the traditionally used end-tidal PCO2 (PETCO2).
Additionally, anesthesiologists, pulmonologists and critical care physicians are now beginning to consider another measurable variable known as “pulmonary carbon dioxide elimination per breath ({dot over (V)}CO2,br).” {dot over (V)}CO2,br is arrived at by multiplication and integration of the airway flow and PCO2 of the respiratory gas over all four phases of the respiratory cycle.
Also, there is growing acceptance of a technique known as indirect calorimetry (e.g., the measurement and/or computation of CO2 elimination and O2 uptake) during anesthesia or mechanical ventilation for the rapid detection of various untoward states such as metabolic upset (e.g. onset of anaerobic metabolism) or pulmonary embolism.
The measurement of pulmonary carbon dioxide elimination ({dot over (V)}CO2), pulmonary oxygen uptake ({dot over (V)}O2) and other indirect calorimetric measurements are facilitated by sampling of mixed respiratory gas. Such sampling of mixed respiratory gas may be accomplished in several ways. One way is to attach a collection vessel such as a bag to the ventilation circuit to collect expired respiratory gas over a period of time. This collection technique is time consuming and of limited value because the collected mixture of respiratory gas is obtained from only one location in the ventilation circuit (e.g., from the expiratory flow conduit). Another technique for sampling mixed respiratory gas is through use of an in-line bymixer device. The bymixer devices of the prior art have been constructed to continually divert a portion of respiratory gas flowing through a conduit into a reservoir. Sanjo, Y., Ikeda, K., A Small Bypass Mixing Chamber for Monitoring Metabolic Rate and Anesthetic Uptake, J. Clin. Monit. 1987; 3: 235-243; Breen P. H., Serina E. R., Bymixer Provides On-Line Calibration of Measurement of Volume Exhaled Per Breath, Ann. Biomed. Eng. 1997; 25:164-171. However, such prior art bymixers were typically difficult to construct and thus somewhat expensive. Also, the gas collection reservoirs of such prior art bymixers were of constant volume and the gas diverting tubes were of constant dimensions and, thus, could not be rapidly adapted or adjusted to accommodate patients of varying size (e.g., small pediatric patients and large adult patients) or changes that may occur in a particular patient's ventilation parameters or clinical status. Finally, the gas collection reservoirs of the prior art bymixers were prone to collect condensed water vapor and respiratory debris and were difficult to clean.
Accordingly, there remains a need in the art for the development of a new bymixer device that is simple and economical to use and is adjustable or adaptable so as to be useable in patients of varying size (e.g., small pediatric patients and large adult patients) and to optimize the continuing measurements made during a given procedure as changes occur in the ventilation circuit and/or in a patient's ventilation parameters or clinical status.